CELTA does not expect you to know all about grammar. In fact, most native speakers from just a few years younger than you will never have studied it at school and will be even more worried than you.
CELTA will, however, train you to plan ahead what you are teaching, then go off and learn/brush up on the area of language the night before you teach it; and assessed lessons do allow for you you not know something and tell students you'll check that and tell them the next day.

The main thing for CELTA is that it is super-intense. Many people drop off it or have a bad time because they had not realised how intense the course is and how much time-management is a factor.